With the continuous development of the semiconductor fabrication technologies and the continuous shrinking of the technical node, to obtain a desired threshold voltage and improve the performance of semiconductor devices, the gate-last technique has been widely used. However, when the critical dimension of the semiconductor device is further reduced, the structure of the conventional Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is unable to match the desired performance requirements even the gate-last process is used. Thus, multiple-gate devices, such as Fin field effect transistors (FinFETs), etc., have attracted extensive attentions.
During the fabrication of FinFETs, it needs to adjust the threshold voltages of the FinFETs to match certain requirements of a practical integrated circuit (IC). One approach to adjust the threshold voltages of the FinFETs is to dope the channel regions of the FinFETs. The widths of the fins of the FinFETs are relatively small. Thus, it needs to dope the fins with a relatively high doping concentration to obtain the desired threshold voltages. However, performing a doping process with the relatively high doping concentration in the fins would affect the carrier mobility of the channel regions of the FinFETs. Further, under a same doping concentration, the variation of the doping concentrations within the channel regions would cause threshold voltage mismatches between different FinFETs.
Another approach to adjust the threshold voltages of the FinFETs is to form work function layers in metal gates of the FinFETs. By adjusting the work functions of the metal gates by changing metal gate stack, the threshold voltages of the FinFETs are able to be adjusted. However, the required work functions of P-type FinFETs and N-type FinFETs are different. Thus, it needs to form different metal gate stack for the P-type FinFETs and the N-type FinFETs, respectively. For similar P-type FinFETs or similar N-type FinFETs, to obtain FinFETs with different threshold voltages, it needs different fabrication processes to form different work function layers. Thus, the fabrication process is complex; and the fabrication process is difficult to control.
The disclosed device structures and methods are directed to solve one or more problems set forth above and other problems.